https://leader.pubs.asha.org/article.aspx?articleid=1912341This Is Your Brain … This Is Your Brain on NoiseProlonged exposure to loud noise can make it more difficult for the brain to process speech and distinguish speech sounds, according to neuroscientists at The University of Texas at Dallas. In a paper published July 28 in Ear and Hearing, they demonstrated for the first time how noise-induced hearing loss ...2014-10-01T00:00:00Research in Brief

Prolonged exposure to loud noise can make it more difficult for the brain to process speech and distinguish speech sounds, according to neuroscientists at The University of Texas at Dallas. In a paper published July 28 in Ear and Hearing, they demonstrated for the first time how noise-induced hearing loss affects the brain’s recognition of speech sounds.

Before the study, scientists had not clearly understood the direct effects of noise-induced hearing loss on the brain’s response to speech. To simulate two types of noise trauma, scientists exposed rats to moderate or intense levels of noise for an hour. One group heard a high-frequency noise at 115 decibels inducing moderate hearing loss, and a second group heard a low-frequency noise at 124 decibels causing severe hearing loss. For comparison, the maximum output of an MP3 player or the sound of a chainsaw is about 110 decibels, and the siren on an emergency vehicle is about 120 decibels.

Researchers, led by Amanda C. Reed, observed how the two types of hearing loss affected speech sound processing in the rats by recording the neuronal response in the auditory cortex a month after the noise exposure. The auditory cortex—one of the main areas that processes sounds in the brain—is organized on a scale, like a piano. Neurons at one end of the cortex respond to low-frequency sounds, while other neurons at the opposite end react to higher frequencies.

In the rats with severe hearing loss, less than one-third of the tested auditory cortex sites that normally respond to sound reacted to stimulation. The sites that did respond showed unusual activity patterns: The neurons reacted slower, the sounds had to be louder and the neurons responded to frequency ranges narrower than normal. Additionally, the rats could not tell the speech sounds apart in a behavioral task they could complete successfully before the hearing loss.

In the group with moderate hearing loss, the area of the cortex responding to sounds didn’t change, but the neurons’ reaction did. A larger area of the auditory cortex responded to low-frequency sounds. Neurons reacting to high frequencies needed more intense sound stimulation and responded slower than those in animals with normal hearing. Despite these changes, the rats were still able to discriminate the speech sounds in a behavioral task.